[New-bugs-announce] [issue27197] mock.patch interactions with "from" imports

[Clark Breyman]

New submission from Clark Breyman:

Unclear if this is a code bug or a gotcha that should be documented:
Cross posted to https://github.com/testing-cabal/mock/issues/365

Since "from"-style imports in modules bind names on the first execution of the import, there is a subtle interaction with patching:

_If a name is imported when the source is patched, the imported name will refer to the patched version even after the source is restored_

Test case:
```python
# patchbug/__init__.py
#
```

```python
# patchbug/a.py
#
class A(object):
    def name(self):
        return "unpatched"
```

```python
# patchbug/b.py
#
from patchbug.a import A

def reference():
    return A().name()

# patchbug/tests.py
#
import mock

"""
patchbug.reference.UnpatchedClass is bound to the value of patchbug.source.UnpatchedClass
at the time of the first import of patchbug.reference. If patched at that time it is not repaired.
"""

def test_unpatched():
    import patchbug.a
    assert patchbug.a.A().name() == "unpatched"

"""
uncommenting the import causes reference.UnpatchedClass to be bound before the patch, fixing
test_reference and breaking test_unpatched
"""
def test_import_reference():
#    import patchbug.b
    pass

def test_patched():
    with mock.patch('patchbug.a.A') as P:
        import patchbug.a, patchbug.b
        P.return_value.name.return_value = "patched"
        assert patchbug.a.A().name() == "patched"
        assert patchbug.b.reference() == "patched"

def test_reference():
    import patchbug.b
    assert patchbug.b.reference() == "unpatched"
```

----------
components: Library (Lib)
messages: 267114
nosy: clarkbreyman
priority: normal
severity: normal
status: open
title: mock.patch interactions with "from" imports
type: behavior
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27197>
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